Microscale combustor-heat exchanger

ABSTRACT

A miniaturized power generation device and method are provided. In one configuration a microscale combustor and heat exchanger may include several repeating unit cells each of which performs combustion, recuperation, and heat exchange. Catalytic combustion may occur inside at least one combustion and one recuperator channel. Specific features may be added to reduce heat loss and pressure drop.

RELATED APPLICATIONS

This application claims the benefit of priority of U.S. Provisional Application No. 61/842,547, filed on Jul. 3, 2013, the entire contents of which application(s) are incorporated herein by reference.

GOVERNMENT LICENSE RIGHTS

This invention was made with government support under DE-FC36-09GO19005 awarded by United States Department of Energy. The government has certain rights in this invention.

FIELD OF THE INVENTION

The present invention relates generally to a miniaturized power generation device, and more particularly, but not exclusively, to a microscale combustor and heat exchanger (μCHX) that may include several repeating unit cells each of which performs combustion, recuperation, and heat exchange.

BACKGROUND OF THE INVENTION

Advances in fabrication technology have helped spur research in miniaturized thermal devices for distributed power generation and heat and mass transport process intensification. These devices have the advantages of being lightweight and compact, thereby yielding high energy and power densities. With a decrease in channel dimensions, the surface area per unit volume of fluid flowing in a microchannel increases thereby increasing the heat transfer rate. The increased surface area per unit volume also enhances completion of heterogeneous surface chemical reactions within microchannels. Therefore, catalytic combustion and heat transfer in microchannels can result in increased efficiency and reduced size of thermal power generation and exchange devices. In addition, the rate of heat generation from combustion and device temperatures can be controlled to some extent by tailoring heterogeneous reactions to occur at specific locations within the channel walls. Thus, high temperatures that cause the formation of NO_(X) can potentially be mitigated. Because of these advantages, several microscale combustor geometries for different fuels have been presented in literature over the past decade. As the combustion channel size decreases, homogeneous (gas-phase) reactions face thermal and radical quenching. Depending on the heat loss and the gas mixture velocity, flame extinction of gas phase hydrogen combustion can occur in channels as large as 1000 μm. Combustion in channels smaller than this limit can occur via heterogeneous (surface) reactions that are promoted by catalysts. Heterogeneous combustion can further trigger homogeneous combustion in the bulk of the fluid.

Combustion within microchannels has been documented in several numerical and experimental studies. For example, Boyarko et al. tested hydrogen-oxygen mixture combustion in a platinum microtube and found that the 400 μm and 800 μm tubes used in their experiments were below quenching size under most atmospheric pressure test conditions. (Boyarko, G. A., Sung, C. J., and Schneider, S. J., 2005, “Catalyzed combustion of hydrogen-oxygen in platinum tubes for micro-propulsion applications,” Proc. Combust. Inst., 30, pp. 2481-2488.) In both numerical simulations as well as experiments, Boyarko et al. observed that there was a minimum threshold heat flux necessary for ignition. When the ignition heat flux was increased further, the gas temperature got so high that a choked flow resulted inside the tube. Zhou et al. modeled conjugate heat transfer within a 500 μm channel made of different wall materials (quartz glass, alumina ceramic, copper) to investigate the effect of wall thermal conductivity on homogeneous/heterogeneous combustion of hydrogen-air mixture. (Zhou, J., Wang, Y., Yang, W., Liu, J., Wang, Z., and Cen, K., 2009, “Combustion of hydrogen-air in catalytic micro-combustors made of different material,” Int. J. Hydrog. Energy, 34, pp. 3535-3545.) They observed that heterogeneous reaction became dominant as thermal conductivity of the material increased. Chen et al., who simulated combustion in a 1-mm channel with different wall materials, reached a similar conclusion. (Chen, G. B., Chen, C. P., Wu, C. Y., and Chao, Y. C., 2007, “Effects of catalytic walls on hydrogen/air combustion inside a micro-tube,” Appl. Catal. A-Gen., 332, pp. 89-97.) They found that heterogeneous reaction was dominant in the beginning of the tube, followed by homogeneous reaction downstream of the tube. Lower wall conductivity was observed to lead to a larger temperature gradient on the surface causing homogeneous combustion to shift upstream. At the highest studied velocity (20 m/s), no homogeneous reaction was observed in channel heights lower than 200 μm in diameter. In a follow-on study, Chen et al. investigated the effect of differences in catalyst configurations in the same geometry as that used in their previous study. (Chen, G. B., Chao, Y. C., and Chen, C. P., 2008, “Enhancement of hydrogen reaction in a micro-channel by catalyst segmentation,” Int. J. Hydrog. Energy, 33, pp. 2586-2595.) A multi-segment catalyst was compared with a single segment catalyst of the same total length. The multi-segment catalyst configuration showed better conversion due to the occurrence of homogeneous reaction in the regions between segments. Karagiannidis and Mantzaras used a 2-D model to simulate transient hetero-homogeneous combustion of methane over platinum catalyst within a 1000 micrometer channel. (Karagiannidis, S., and Mantzaras, J., 2010, “Numerical investigation on the start-up of methane-fueled catalytic microreactors,” Combust. Flame, 157, pp. 1400-1413.) For the pressures in the range of 1 bar-5 bar, they found that ignition and steady state microreactor residence times decreased with an increase in pressure. Combustors with lower thermal conductivity walls had smaller ignition times.

Recuperation has been used alongside combustion in order to preheat the gas mixture by several groups. Lloyd and Weinberg fabricated a spiral counterflow combustor, often referred to as a “Swiss roll” type combustor to improve the efficiency of combustion processes. (Lloyd, S. A., and Weinberg, F. J., 1974, “A burner for mixtures of very low heat content,” Nature, 251 (5470), pp. 47-49.) Peterson et al. developed a microscale hydrogen combustor with counterflow heat recuperator. (Peterson, R. B., and Vanderhoff, J. A., 2000, “A catalytic combustor for microscale applications,” Combust. Sci. Technol. Comm., 1, pp. 10-13.) They observed that preheating helped to keep a sustained homogeneous reaction. In addition to a microscale combustor, an efficient heat exchanger is required in order to transfer heat produced by the reaction to a working fluid. The heat transfer to the working fluid will alter the wall temperature distribution, which will in turn affect the combustion process. Janicke et al. used hydrogen combustion over a platinum covered surface to heat a gas stream cross-flow to the combustion gas flow in a microscale heat exchanger. (Janicke, M. T., Kestenbaum, H., Hagendorf, U., Schüth, F., Fichtner, M., and Schubert, K., 2000, “The controlled oxidation of hydrogen from an explosive mixture of gases using a microstructured reactor/heat exchanger and Pt/Al2O3 catalyst,” J. Catal., 191 (2), pp. 282-293.)

There have been several studies involving stacked microchannel arrays for various applications. The review articles by Fan and Luo and Khan and Fartaj provide some of the recent examples of stacked microchannel devices including heat exchangers and chemical reactors. (Fan, Y., and Luo L., 2008, “Recent applications of advances in microchannel heat exchangers and multi-scale design optimization,” Heat Transf. Eng., 29(5), pp. 461-474. Khan, M. G., and Fartaj, A., 2011, “A review on microchannel heat exchangers and potential applications”, Int. J. Energy Res., 35, pp. 553-582.) In stacked microscale reactors, one layer could have several parallel microchannels wherein a reaction occurs while exchanging heat with a working fluid that flows in an adjacent layer. Such an arrangement has been used for methane steam reforming where a fuel combusts in the combustor layers and transfers the produced heat to the reformer sheets. Ryi et al. tested methane steam reforming with hydrogen catalytic combustion in an integrated microchannel reactor. (Ryi, S. K., Park, J. S., Choi, S. H., Cho, S. H., and Kim S. H., 2005, “Novel micro fuel processor for PEMFCs with heat generation by catalytic combustion,” Chem. Eng. J., 113, pp. 47-53.) The designed device consisted of cover plate, a base plate and 50 plates (25 alternating combustor and reformer plates) with microchannels. Inconel plates were used to fabricate the microchannel sheets and stainless steel sheets were used for the cover and base plates. Each sheet had 22 microchannels in parallel with 500 μm in diameter, 250 μm in depth and 17 mm in length. Pt—Sn/Al₂O₃ and Rh—Mg/Al₂O₃ were impregnated by wash-coating in the combustor and reformer for catalytic reactions respectively. Hwang et al. developed a similar combined combustor and methane reformer device and were able to achieve 95% conversions and hydrogen production rate of 0.78 mol/h in the reformer. (Hwang, K. R., Lee, C. B., Lee, S. W., Ryi, S. K., and Park, J. S., 2011, “Novel micro-channel methane reformer assisted combustion reaction for hydrogen production,” Int. J. Hydrog. Energy, 36, pp. 473-481.) Their device consisted of a variety of chemically etched metal plates, such as half-etched straight channel plates (10 sheets), fully etched 3D mixing channel plates (2 sheets), and cover/holder/separator plates (5 sheets). Hydrogen and/or methane were used as the fuel in the combustor sheets to provide heat for methane reformation. A Pt-coated mesh catalyst was used as an igniter at the inlet of the combustor until a flame was generated.

Mettler et al. used CFD simulations to model stacks of different sizes and characterize the effects of scaling up of microchemical systems. (Mettler, M. S., Stefanidis, G. D., and Vlachos, D. G., 2011, “Enhancing stability in parallel plate microreactor stacks for syngas production,” Chem. Eng. Sci., 66, pp. 1051-1059.) They studied syngas production from methane using a parallel-plate reactor with alternating combustion and steam reforming channels. The author compared stacks of 3 units to 15 units, each comprised a combustion channel and reformer channel. They found that heat losses caused extinction of combustion in the outer channels and consequently reduced the efficiency of the smaller stack. Whereas extinction of combustion occurred in the outer channels even for the larger stack, the interior channels sustained combustion, resulting in a higher efficiency. They also recommended stack materials with thermal conductivities higher than 100 W/m-K for a more stable device.

Very recently Zhang et al. synthesized a Pt-based catalyst, and investigated the behavior of hydrogen catalytic combustion at low temperatures of the hydrogen/dry air mixture. (Zhang, C., Zhang, J., and Ma, J., 2012, “Hydrogen catalytic combustion over a Pt/Ce0.6Zr0.4O2/MgAl2O4 mesoporous coating monolithic catalyst,” Int. J. Hydrog. Energy, 37, pp. 12941-12946.) They found that for low temperature catalytic combustion of hydrogen, the initial reaction temperature, H₂ concentration, and flow rates were very important parameters. They tried hydrogen combustion at mixture temperatures of 298 K and 263 K and their results show that higher H₂ concentration was helpful in initiating and sustaining catalytic combustion. For the 263 K combustion, the authors could not achieve conversions higher that 40% for low hydrogen concentrations and although they could start the catalytic combustion, they described the largest challenge to be avoiding product water from freezing.

Additionally, several reaction mechanisms are available in literature on hydrogen oxidation. Although the rates were determined for macroscale channels, these reaction rate coefficients can be used for microscale simulations since they are surface reactions. Warnatz et al. studied stagnation flow of hydrogen-oxygen mixture over a platinum surface and developed a reaction mechanism for H₂/O₂ combustion. (Warnatz, J., Allendorf, M. D., Kee, R. J., and Coltrin, M. E., 1994, “A model of elementary chemistry and fluid mechanics in the combustion of hydrogen on platinum surfaces,” Combust. Flame, 96, pp. 393-406.) Warnatz et al.'s as well as three other homogeneous reaction mechanisms as well as three heterogeneous reaction schemes were tested by Appel et al. (Appel, C., Mantzaras, J., Schaeren, R., Bombach, R., Inauen, A., Kaepperli, B., Hemmerling, B., and Stampanoni, A., 2002, “An experimental and numerical investigation of homogeneous ignition in catalytically stabilized combustion of hydrogen/air mixtures over platinum,” Combust. Flame, 128, pp. 340-368.) When combustion in a 7 mm high channel with platinum covered walls was considered, they found differences from 8 to 66 percent between the modeling results using these schemes and their own experimental results for ignition characteristics. Their study showed that Warnatz's homogeneous reaction mechanism and Deutschmann's heterogeneous reaction mechanism give the best predictions, within 8% of the experimental results. (Deutschmann, O., Schmidt, R., Behrendt, F., and Warnatz, J., 1996, “Numerical modeling of catalytic ignition,” Proc. 26th Symposium (International) on Combustion/The Combustion Institute, Pittsburgh, Pa., pp. 1747-1754.)

However, a need remains in the art for a compact and efficient microchannel heat exchangers for low temperature applications, in particular, for example, ones which provides combustion of fuels and heat exchange from the combustion gases to a working fluid.

SUMMARY OF THE INVENTION

In one of its aspects the present invention relates to a miniaturized power generation device, such as a microscale combustor and heat exchanger (μCHX). The μCHX may include several repeating unit cells each of which performs three unit operations: combustion, recuperation, and heat exchange. Heterogeneous catalytic combustion may occur on the walls of microchannels in the presence of a platinum catalyst. In one particular configuration, the present invention may include a distributed catalyst arrangement which deters extinction of the reaction due to a cold gas stream and which provides a high hydrogen conversion (greater than 95 percent) for a range of operating conditions.

For instance, in one of its aspects, the present invention may provide several microchannels that are connected in parallel in order to meet the thermal power requirements of the desired application. The parallel microchannels may be linked together by inlet and outlet headers that distribute the flow uniformly amongst the microchannels. When only one working fluid is involved, a single layer of parallel microchannels could be sufficient, as in the case of a heat sink or simple chemical reactors. However, for heat or mass exchangers and more complex chemical reactors, a stacked up, multi-layer parallel microchannel architecture may be needed. In the design of such microchannel devices, it may be sufficient to optimize the performance of a single microchannel “unit cell” (for example, two microchannels separated by a non-permeable wall if the device is a heat exchanger) and to ensure that the flow distribution between the microchannel unit cells and between the stacked layers is uniform. Typically, the unit cells and headers may also be designed with pressure drop constraints in mind.

In one exemplary application, the devices and methods of the present invention may find use in an automotive cryo-adsorbant storage system for hydrogen. In such an exemplary application, hydrogen gas that exits a cryo-adsorbant storage tank needs to be heated to a minimum temperature of 233K (−40° C.) prior to entering the fuel cell. During cold start conditions, heat exchange with ambient air or with the fuel cell coolant is insufficient to provide this minimum temperature, thereby requiring an additional source of thermal energy. This thermal energy can be provided by combusting a small portion of a cold hydrogen stream in a device capable of transferring heat of reaction back to the cold stream. To maintain a high on-board efficiency and low storage system weight and volume, it is desirable for the device to be small, lightweight and operate at a high efficiency. Another exemplary application is building or distributed heating. Exemplary configurations of compact devices based on parallel microchannel architecture are presented herein.

For example, the present invention may provide a microscale combustor and heat exchanger, comprising a plurality of layers each having one or more respective channels extending therethrough. The layers may be joined to one another to permit gaseous communication between selected respective channels of the layers. The plurality of layers may include a combustor layer comprising at least one combustion channel having a catalyst disposed therein and a recuperator layer comprising at least one recuperation channel having a catalyst disposed therein. The recuperation channel may be disposed in gaseous communication with a respective combustion channel to receive a combustion gas therefrom. The combustor layer and recuperator layer may be disposed in stacked arrangement so that the at least one recuperation channel and the at least one combustion channel are disposed over one another with a common channel wall therebetween. The plurality of layers may also include a heat exchange layer having at least one heat exchange channel disposed therein, the recuperation layer may be disposed between the heat exchange layer and the combustor layer.

In addition, the present invention may provide a microscale combustor and heat exchanger comprising a plurality of layers each having one or more respective channels extending therethrough. The plurality of layers may include a combustor layer having at least one combustion channel disposed therein, and a heat exchange layer having at least one heat exchange channel disposed therein. In addition, a casing may be provided around and enclosing the plurality of layers and may include an inner wall defining a cavity disposed therein; the cavity may be dimensioned to provide a gap between at least a portion of the inner wall and the plurality of layers, with at least one heat exchange channel disposed in gaseous communication with the gap. A recuperator layer may be provided having at least one recuperation channel disposed therein, and the recuperation layer may be disposed between the heat exchange layer and the combustor layer. The at least one heat exchange channel may include an inlet in gaseous communication with the gap and the heat exchange and combustor layers may each include respective working fluid outlet ports in gaseous communication with the gap. Further, one or more of the plurality of layers may include a groove or two concentric grooves disposed therein between the respective channels and an edge of the respective layer. The at least one combustion channel, at least one recuperation channel, and/or at least one heat exchange channel may include pin fins disposed therein, and the at least one combustion channel and/or at least one recuperation channel may include a catalyst disposed therein.

In yet an additional aspect, the present invention may provide a microscale heat exchanger, comprising a plurality layers each comprising one or more respective channels extending therethrough. The plurality of layers may include one or more heat exchange layers having at least one heat exchange channel disposed therein, and may be disposed in stacked arrangement so that at least one channel from each of two or more layers is disposed adjacent one another with a common wall therebetween through which heat may be exchanged. The plurality of layers may include a shroud disposed between the one or more respective channels and associated respective edges of the layers. One or more of the plurality of layers may include a groove disposed therein between the shroud and an edge of the layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing summary and the following detailed description of exemplary embodiments of the present invention may be further understood when read in conjunction with the appended drawings, in which:

FIG. 1 schematically illustrates a unit cell of an exemplary microscale combustor and heat exchanger (μCHX) in accordance with the present invention;

FIG. 2A schematically illustrates temperature contours within the μCHX unit cell of FIG. 1;

FIG. 2B illustrates hydrogen mass fraction along the length of the combustor and recuperator channels of the μCHX unit cell of FIG. 1;

FIG. 3A-3D schematically illustrate an exemplary combustor shim, an exemplary upper recuperator shim, an exemplary lower recuperator shim, and exemplary heat exchange channel shims, respectively, in accordance with the present invention, each comprising eight channels;

FIGS. 4A-4C schematically illustrate an exploded view of an exemplary physical configuration of eight unit cells, of the type shown in FIG. 1, comprising five different layers, with FIG. 4A illustrating the flow of fuel/air mixture (H₂/air) through the layers, FIG. 4B illustrating the flow of exhaust through the layers, and FIG. 4C illustrating the flow of nitrogen through the layers;

FIG. 5 illustrates velocity contours (scale in m/s) for one half of the microchannels within the combustor layer shown in FIG. 3A;

FIG. 6 schematically illustrates an exemplary header in accordance with the present invention for use with stacked unit cells of the type shown in FIGS. 4A-4C in which the location of inlets and outlets is shown;

FIGS. 7A, 7B schematically illustrate exemplary top layers of H₂/air and nitrogen headers in accordance with the present invention showing the transition from circular tubing to respective plena;

FIG. 8 illustrates pressure contours (Pa) of combustible gas flow through the header of FIG. 6 in combination with the layers of FIG. 4B;

FIG. 9 illustrates pressure contours (Pa) of the nitrogen flow through the header of FIG. 6 in combination with the layers of FIG. 4A;

FIG. 10 schematically illustrates an exploded view of a further exemplary configuration of a microscale combustor and heat exchanger in accordance with the present invention comprising five layers and two unit cells having pin fins and showing the flow direction of a working fluid, a fuel/air mixture, and exhaust across one half of each layer;

FIG. 11A schematically illustrates the combustor layer of FIG. 10;

FIG. 11B schematically illustrates the upper recuperator layer of FIG. 10;

FIG. 11C schematically illustrates the lower recuperator layer of FIG. 10;

FIG. 11D schematically illustrates the heat exchange layer of FIG. 10;

FIGS. 12A, 12B schematically illustrate a groove without holes and a groove with through holes, respectively;

FIG. 13 schematically illustrates a partial cross-sectional view of the combustor layer of FIG. 11A taken along the sectioning line 13-13;

FIGS. 14A, 14B schematically illustrate velocity contours (scale in m/s) for one half of the microchannels within the combustion and heat exchange layers, respectively, of the microscale combustor and heat exchanger of FIG. 10;

FIGS. 15A, 15B schematically illustrate velocity profiles for combustion and heat exchange layers, respectively, at two different longitudinal locations indicated by the dashed lines in FIGS. 14A, 14B;

FIG. 16 schematically illustrates an exemplary header in accordance with the present invention for use with stacked unit cells of the type shown in FIG. 10 in which the location of inlets and outlets is shown;

FIG. 17 schematically illustrates assembly of the stacked unit cells of the type shown in FIG. 10 along with a casing;

FIG. 18 illustrates pressure drop in the heat exchanger side (in Pa) of the device of FIG. 17, with one quarter of the flow paths shown;

FIG. 19 illustrates velocity vectors at the mixing region of the device of FIG. 17;

FIG. 20 schematically illustrates an exploded including the top and bottom distribution layers of yet a further exemplary microscale combustor and heat exchanger in accordance with the present invention;

FIG. 21 schematically illustrates a combustor layer of the device of FIG. 20 comprising 2 unit cells;

FIG. 22 illustrates a combustor shim, corresponding to the combustor layer design of FIG. 3A, as fabricated by chemical etching and laser cutting;

FIG. 23 schematically illustrates a test facility used for testing a microscale combustor and heat exchanger of the present invention;

FIG. 24 illustrates the measured variation of H₂ conversion, efficiency, and heat loss ratio by residence time of a fabricated prototype;

FIG. 25 illustrates the measured variation of H₂ conversion, efficiency, and heat loss ratio by body temperature of a fabricated prototype; and

FIGS. 26A, 26B schematically illustrate a microchannel of a microscale combustor in accordance with the present invention comprising an electric heater configured to heat a selective location of the combustor, where the electric heater can be as small as a wire (FIG. 26A) or as large as the size of the combustor itself (FIG. 26B).

DETAILED DESCRIPTION OF THE INVENTION

In one of its aspects the present invention provides a general device design which includes several repeating unit cells 100 each of which may perform three unit operations: combustion, recuperation, and heat exchange, FIG. 1. Heat from combustion may be transferred to a working fluid that can either be a gas (air) or liquid (water). Heterogeneous catalytic combustion may occur on the walls of microchannels 110, 140 in the presence of a platinum catalyst 130, 132.

Two levels of numerical simulations are performed to realize the design. The first level represents a single unit cell 100 comprising a combustion channel 110, two recuperator channels 140, and two heat exchange channels 120, FIG. 1. At the unit cell level, a two-dimensional numerical model with detailed surface chemistry is used to realize a design with a high unit cell thermal efficiency. It is shown that with the help of a novel distributed catalyst arrangement 130, 132, extinction of the reaction due to the cold gas stream is prevented and a high hydrogen conversion (greater than 95 percent) is achieved for a range of operating conditions. The second level of simulations is at the physical, device scale, comprising multiple unit cells connected together with appropriate fluidic headers FIGS. 3A-7B. Multiple unit cells are created from several layers 410, 420, 300, 430, 440 (hereafter “410-440”), with multiple such unit cells then stacked in parallel with appropriate headers to provide a multi-unit μCHX 600, FIGS. 4A-4C, 6. Three dimensional simulations of fluid flow are performed to ensure uniformity in flow distribution while maintaining low pressure drop through the μCHX 600, FIGS. 8, 9. Fabrication constraints are incorporated into the device level design and simulations.

In one exemplary application, the multiple unit cell microscale combustor and heat exchanger (μCHX) 600 can operate at temperatures as low as 200 K. One particularly useful application of the μCHX 600 is cryo-adsorbent hydrogen storage systems for fuel cell cars under cold start conditions. In this exemplary application, desired operating conditions for the μCHX 600 are shown in Table 1. A fraction of the incoming cold hydrogen gas would be premixed with air to provide the thermal energy rate needed to increase the temperature of the rest of the hydrogen flow from 200 K to 233 K. The hydrogen flow rate to the fuel cell could vary between 0.5 g/s to 2 g/s and the working pressure could vary between 5 to 20 bars. For the cold start condition, the environment, from which air is drawn for the combustion process, is assumed to be at 233 K (−40° C.).

TABLE 1 Desired operating conditions for the μCHX P_(H2,in) m P_(c),_(in) T_(H2,in) (K) T_(H2,out) (K) (bar) (g/s) T_(c,in) (K) (bar) 200 233 5-20 0.5-2 233 1

Unit Cell Level Design

Each unit cell 100 of the multi-unit μCHX 600 was designed to perform, at the minimum, the unit operations of combustion and heat exchange to the working fluid (e.g., a cold hydrogen stream), by having a combustion channel 110 surrounded on both sides by heat exchange channels 120. The combustion channel 110 may include inner catalyst beds 130 disposed on respective inner surfaces 112 of the combustion channel 110, and may include outer catalyst beds 132 disposed on respective outer surfaces 114 of the combustion channel 110, which catalysts beds 130, 132 may include a noble metal, such as platinum, palladium, rhodium, and/or other suitable material, for example. The catalyst can be in the form of a thin layer (coating) deposited on the channels walls, or the catalyst can be in the form of insets put inside the channels (or attached to the walls). The catalyst (coating or insert) may comprise a porous material.

Results from simulations (described below) for such a unit cell design showed that the flow of a very cold working fluid (e.g., 200 K hydrogen gas) in the heat exchange channels 120 reduced the temperature of the catalyst beds 130, 132 and prevented catalytic combustion. Therefore, recuperator channels 140 were provided between the combustion and heat exchange channels 110, 120 to provide a thermal buffer therebetween, FIG. 1. Thus, the unit cell 100 may include a central combustor microchannel 110 surrounded by two recuperator channels 140 on either side. The heat exchange channels 120, in turn, may be provided on either side of the recuperator channels 140. In addition to providing a thermal buffer, the recuperator channels 140 also permit recuperator gases disposed therein to pre-heat the incoming air-fuel (e.g., air-H₂) mixture prior to exiting the unit cell 100, thereby providing a region of recuperation “R”, FIG. 1. (The preheat region “R” optionally may or may not exist based on the application.) The added length to the recuperator channels 140 was adjusted based on the simulation results so that the recuperator gases remain at the minimum temperature of 373 K (100° C.)—a requirement that was imposed in order to avoid condensation.

Geometrical arrangements and thermofluidic parameter values that ensure high efficiency and conversion were determined Efficiency is used to identify the overall performance of the unit cell 100, and is defined as the ratio of the amount of heat transferred to the working fluid (e.g., cold hydrogen stream) to the chemical energy of input hydrogen in the combustible gas mixture,

$\begin{matrix} {\eta = \frac{{{\overset{.}{m}}_{wf}\left( {h_{out} - h_{in}} \right)}_{wf}}{{\overset{.}{m}}_{H_{2}}\Delta \; {h_{reaction}/M_{H_{2}}}}} & (1) \end{matrix}$

where M_(H) ₂ is the molar mass of hydrogen, Δh_(reaction) is the molar enthalpy of reaction, and {dot over (m)}_(H) ₂ and {dot over (m)}_(wf) are the inlet hydrogen and working fluid mass flow rate, respectively. Enthalpy of reaction is calculated at the volumetrically-averaged temperature in the catalyst section of the combustor. Hydrogen conversion is defined as the ratio of the amount of hydrogen combusted to that of input hydrogen,

$\begin{matrix} {{Conversion}_{H_{2}} = {\frac{Y_{H_{2,}{in}} - Y_{H_{2,}{out}}}{Y_{H_{2,}{in}}}.}} & (2) \end{matrix}$

As part of the design considerations, to produce a high specific power within the unit cell 100, an equivalence ratio of unity was considered. The equivalence ratio, φ, is the ratio of the molar fuel-to-air ratio at the desired test conditions to that at stoichiometric conditions. The large equivalence ratio also results in reduced pressure drop for the same thermal power generated when compared with lower equivalence ratio mixtures.

In the exemplary design, the height of the combustion and heat exchange channels 110, 120 were 300 μm each, and height of the recuperator channel 140 was 150 μm. The width of all channels 110, 120, 140 in the unit cell was 2 mm while the length, L, of the unit cell 100 was kept at 15 mm. Based on simulations, this length L was found to provide sufficient area for heat exchange between the recuperator and heat exchange channels 140, 120 while keeping the pressure drop low. All outer walls of the recuperator and heat exchange channels 140, 120 were considered insulated (as indicated by the cross-hatching in FIG. 1).

Initially the catalyst beds 130, 132 were located entirely on the inner walls 112 of the combustion channel 110 alone; however, simulations showed that almost half of the catalyst length was not being efficiently utilized because of the low reactant mixture temperature. Despite the thermal buffer provided by the recuperator channels 140, the cold hydrogen gas flow in the heat exchange channels 120 tended to decrease the mixture temperature rapidly past the initial part of the catalyst bed 130. As a result, hydrogen conversion of only around 80 percent was typically achieved.

In order to obtain higher conversions while managing the amount of catalyst used, a novel catalyst bed arrangement was used. The basic premise of the new arrangement was that higher gas mixture and catalyst bed temperatures resulted in higher reaction rates and hence more complete hydrogen conversion. To achieve this larger catalyst temperature, the catalyst bed was redistributed such that ⅖ of the catalyst bed was shifted from the end of the inner wall 112 of the combustion channel 110 and placed in the recuperator channels 140 on the outer surface 114, (i.e., outer catalyst bed 132). The location of the outer catalyst beds 132 within the recuperator channels 140 coincided with the location of the inner catalyst beds 130 in the combustion channel 110, FIG. 1. Simulations showed that the optimum ratio for the length of the inner catalyst bed 130 to that of the outer catalyst bed 132 was 1.5 so that ⅗ of the length was located in the combustion channel 130 and ⅖ of the combined bed length was located in the recuperator channel 140, though other ratios may be used in accordance with the present invention. For example, the combustion channel 110 may have larger catalyst area than the recuperator channel 140 or vice versa, and the ratio of the catalyst area in each channel 110, 140 over the total catalyst area can be in the range of 1/10th to 9/10th. With this new ⅖ arrangement of catalyst beds 130, 132, hydrogen conversions of about 99% were achieved with the L=15 mm and a total catalyst length of 12.5 mm for each inner/outer bed 130, 132 pair. Depending on the application and design, the optimum ratio may be different.

The catalyst used in a microscale combustor and heat exchanger 600 in accordance with the present invention may have any site density. Catalyst site density is defined as the amount of catalytically active site per unit area (cm²). Catalyst site density may be adjusted based on the required reactivity of the catalyst, e.g., the lower the site density, the lower the reaction rate (and therefore produced power). In addition, the catalyst in the combustor and recuperator channels 110, 140 may have different site densities. In the case where the catalyst surface (catalyst bed) in the combustor and recuperator channels 110, 140 have different site densities, the total amount of catalyst can be calculated by multiplying the site density value by the area. The ratio of the amount of catalyst in each channel 110, 140 over the summation of the catalyst sites (total catalyst amount in the combustion channel 110+total catalyst amount in the recuperator channel(s) 140) can be in the range of 1/10 to 9/10.

Numerical Simulations of the Unit Cell

Two-dimensional, steady-state simulations were carried out on the mass, momentum, energy and species (both gas-phase and surface species) balance equations for the unit cell geometry indicated in FIG. 1. The reactant gases entering the channel 110 were modeled as comprised of a hydrogen and dry air (approximated as a 21 percent by volume oxygen and 79 percent by volume nitrogen) mixture. Specifically, combustion of hydrogen on platinum surface was modeled using the reaction mechanisms and detailed scheme of Deutschmann et al. The scheme, shown in Table 2, includes 7 gas species, 5 surface species and 13 reaction steps, and has shown good comparison with experimental results. The five surface species in this scheme are H(s), O(s), OH(s), H₂O(s) and Pt(s). Pt(s) describes free surface sites that are available for adsorption. A surface site density of Γ=2.7×10⁻⁹ mol/cm² was used simulating polycrystalline platinum, and it was assumed that the catalyst surface was initially covered by Pt(s).

TABLE 2 Surface reactions for hydrogen oxidation on platinum. (Deutschmann et al.) Reaction A (S) β E_(a) 1—H₂ + 2Pt(s) → 2H(s)^(a)  0.046^(b) 0.0 0 2—2H(s) → H₂ + 2Pt(s) 3.7 × 10²¹ 0.0 67.4-6.0 H(s) 3—O₂ + 2Pt(s) → 2O(s) 0.07^(b) 0.0 0 4—2O(s) → O₂ + 2Pt(s) 3.7 × 10²¹ 0.0 213.2-60 O(s) 5—H + Pt(s) → H (s) 1.00^(b) 0.0 0 6—O + Pt(s) → O (s) 1.00^(b) 0.0 0 7—OH + Pt(s) → OH (s) 1.00^(b) 0.0 0 8—H₂O + Pt(s) → H₂O(s) 0.75^(b) 0.0 0 9—H (s) + O(s)

 OH (s) + Pt(s) 3.7 × 10²¹ 0.0 11.5 10—H(s) + OH(s)

H₂O(s) + Pt(s) 3.7 × 10²¹ 0.0 17.4 11—H₂O(s) + O(s)

OH(s) + OH(s) 3.7 × 10²¹ 0.0 48.2 12—OH (s) → OH + Pt(s) 1.0 × 10¹³ 0.0 192.8 13—H₂O(s) → H₂O + Pt(s) 1.0 × 10¹³ 0.0 40.3 ^(a)The hydrogen adsorption (first reaction) is first order with respect to platinum. ^(b)Sticking coefficient.

The mass fraction of species at the inlet of the unit cell 100 was defined by the equivalence ratio. For brevity, the governing equations and boundary conditions were set according to previous work by two of the presently named inventors. (Ghazvini, M., and Narayanan, V., 2011, “Performance characterization of a microscale integrated combustor recuperator oil heat exchanger,” Proc. AJTEC2011: ASME/JSME 2011 8th Thermal Engineering Joint Conference, Honolulu, Hi., 2011). The numerical model was validated against the combined experimental and numerical study on hetero-/homogeneous combustion of hydrogen/air mixtures over platinum in a single channel by Appel et al. (Appel, C., Mantzaras, J., Schaeren, R., Bombach, R., Inauen, A., Kaepperli, B., Hemmerling, B., and Stampanoni, A., 2002, “An experimental and numerical investigation of homogeneous ignition in catalytically stabilized combustion of hydrogen/air mixtures over platinum,” Combust. Flame, 128, pp. 340-368.) The current simulations were seen to predict the experimental data in Appel et al. at least as well as the parity between their own numerical simulations and experiments.

The governing equations and boundary conditions were solved in FLUENT® V14 (ANSYS, Inc., Cannonsburg, Pa., USA) in conjunction with Chemkin-CFD™ (Reaction Design, San Diego, Calif., USA) for the chemical reactions. A non-uniform mesh was used to refine the near-wall regions. The total number of grids was 34,300 for the whole model domain. It was initially verified that an orthogonal staggered grid of 606×20 grid points (in x and y, combustion channel, or 12,120 grids) was sufficient to produce a grid independent solution. Additionally, adaptive mesh refinement was also applied in several locations based on the gradients of mass imbalance for better convergence. The simulation convergence was decided when the residuals approached steady values asymptotically and when the relative residuals were smaller than 10⁻⁷ for continuity, momentum, energy and species, with the exception of residuals for O and H species which were less than 10⁻³.

Based on the required mass flow rate of cold hydrogen in Table 1, the first step in a unit cell simulation was to determine the flow rate of cold hydrogen and flow rate of hydrogen in the combustion channels 110 within each unit cell 100. Based on previous work with oil as the heat exchange fluid, the initial inlet mixture velocity to the combustion channel was picked to be 4 m/s. In that study, this flow rate provided sufficient residence time for conversions in excess of 90 percent. The inlet velocity of the cold hydrogen stream was set arbitrarily to a value. The exit temperature of the cold hydrogen stream was checked against the requirement in Table 1 (>233 K). Simultaneously, the hydrogen conversion was verified to be in excess of 90 percent. If the exit temperature was found to be lower, the flow rate of cold hydrogen was lowered. If the hydrogen conversion was found to be lower, the inlet mixture velocity of the combustion gases was lowered. Once conditions that met both the cold hydrogen exit temperature as well as conversion were achieved, the fraction of hydrogen stream required for combustion to that being heated in the heat exchange channels 120 was determined. Based on the ratio of the maximum mass flow rate of the cold gas in the μCHX to the mass flow rate in each unit cell, a total number of 168 unit cells with the channel width of 2 mm was found to be sufficient to provide heat to 2 g/s of cold hydrogen. For the lower mass flow rate limit of 0.5 g/s in Table 1, the same number of unit cells was retained, with a corresponding decrease in velocity of the cold hydrogen and combustion mixture gases. For example, at a pressure of 5 bar the flow rate of cold hydrogen gas through each of the 168 unit cells were 6.1 mg/s and 1.5 mg/s for a total flow rate of 2 g/s and 0.5 g/s, respectively.

A representative temperature contour plot within the unit cell 100 for the conditions indicated in Table 1 and heat exchange fluid (cold hydrogen) inlet flow rate of 2 g/s, inlet temperature of 200 K, and pressure of 5 bars is shown in FIG. 2A. The temperature contours indicate regions of high temperatures at the catalyst beds 130, 132 due to heterogeneous combustion. Fluid and surface temperatures reached a peak value near the first one-third length of the inner catalyst bed 130 within the combustion channel 110 and then decreased downstream due to the transfer of heat to the recuperator and heat exchange channels 140, 120. For this condition, outlet hydrogen and exhaust gas temperatures were 235.7 K and 391.5 K, respectively. FIG. 2B illustrates the cross-sectional averaged hydrogen mass fraction distribution in the unit cell 100. The location of the catalyst beds 130, 132 in both the combustion and recuperator microchannels 110, 140 are represented in this plot for clarity. Hydrogen/air mixture enters the combustion channel 110 with a hydrogen mass fraction of 0.0283, corresponding with an equivalence ratio of unity. After going through the inner catalyst bed 130 in the combustion channel 110, the mass fraction reduces to 0.0054 indicating that the hydrogen conversion is 81% within the combustion channel 110. The remaining hydrogen reacts in the outer catalyst bed 132 in the recuperator channel 140 such that the mixture exits the unit cell 100 with less than 1% of the initial hydrogen content.

In the unit cell simulations, based on the requirements stated in Table 1, two parameters were varied—the inlet pressure and mass flow rate of the cold hydrogen gas. Table 3 summarizes the achieved conversions and unit cell efficiencies for the extremities in the range of the desired pressures and hydrogen mass flow rates presented in Table 1. It can be seen that the exit temperature in all cases is in excess of 233K and that the conversions and efficiencies are in excess of 99% and 92% respectively. Pressure drop values within the combustor and recuperator channels 110, 140 are also shown in Table 3. A larger cold hydrogen mass flow rate requires a proportionally larger hydrogen/air mixture flow rate, thereby increasing the pressure drop in the combustion and recuperator channels 110, 140 with increase in cold hydrogen mass flow rate. However, pressure drop is almost independent of the working fluid pressure, because the mass flow rate of hydrogen is kept the same for both 5 bar and 20 bar. The slight increase in pressure drop with higher pressure is a result of the larger specific heat value at higher pressures. With the temperature difference between the inlet and exit fixed at 33 K, a larger Cp resulted in an increase in the amount of heat rate needed and hence a slightly larger flow of hydrogen-air mixture within the combustion channel 110.

TABLE 3 Simulation results for the desired pressure and hydrogen mass flow rate ranges shown in Table 1 H₂ conversion (in {dot over (m)}_(H) ₂ P_(H) ₂ T_(out,H) ₂ combustor) η ΔP (g/s) (bar) (K) (%) (%) (Pa) 2 5 235.7 99.8 93.3 5736.3 20 236.1 99.5 92.9 5833.9 0.5 5 237.7 99.8 92.4 991.6 20 243.8 99.4 92.1 1014.4

Numerical Simulations for Comparison to Laboratory Measurements

One goal was to fabricate and characterize the performance of a multi-unit cell μCHX and validate the numerical simulations with laboratory measurements. However, due to safety considerations in the laboratory, cold nitrogen gas was used in place of cold hydrogen gas as the heat transfer fluid. Since density and thermal properties of nitrogen are considerably different from those of hydrogen, additional simulations were performed using cold nitrogen gas for the unit cell 100 of FIG. 1 to create simulation results which could later be compared to laboratory measurements of a prototype device.

There are two thermal resistances in the path for the requisite amount of heat to be transferred within the unit cell 100 from the heat exchange wall 116 (separating the recuperator and heat exchange channels 140, 120) to the cold gas. The first one pertains to the convection resistance, R_(conv)=1/(h_(cold gas)A), while the second one is the resistance due to heating of the cold gas stream, R_(heat)=1/({dot over (m)}Cp). For the range of hydrogen flow rates considered, the flow is laminar and hence the heat transfer coefficient, assuming fully developed flow, is about 1120 W/m²-K. The R_(conv) and R_(heat) for cold hydrogen flow within each unit cell 100 are 35.7 K/W and 5,926 K/W respectively. The R_(heat) estimate is based on a working pressure of 5 bar and for a flow rate of 2 g/s. Since R_(heat) is the dominant thermal resistance in transferring heat to the cold gas, in order to preserve the same representative thermal conditions as cold hydrogen flow, it is clear that the R_(heat) between hydrogen and nitrogen flows has to be matched. Hence, the heat capacity rates (1/R_(heat)) between the cold hydrogen and cold nitrogen flows, as well as the temperature rise (see Table 1) were kept identical. This meant that, because of the high specific heat of hydrogen compared to nitrogen (Cp_(H) ₂ =13.02·Cp_(N) ₂ ), the nitrogen mass flow rate would be proportionally higher than hydrogen mass flow rate. This larger flow rate resulted in a R_(conv) of 35.0 K/W and a R_(heat) of 5,974 K/W for the cold nitrogen stream at a working pressure of 5 bar and flow rate of 26.1 g/s (˜13 times the maximum cold hydrogen flow rate).

Table 4 shows the result of the simulations with nitrogen as the working fluid. The working pressure was fixed at 5 bar for these simulations. Nitrogen mass flow rate of 26.1 g/s and 6.57 g/s have the same heat capacity of 2 g/s and 0.5 g/s of hydrogen, respectively. By a comparison of results in Tables 3 and 4, it can be seen that the hydrogen conversion remains largely unaffected by changing the heat exchange fluid. This result is to be expected since changing the heat exchange fluid only changes the boundary condition on the combustion process. When the heat capacity rates are matched, the temperature drop along the heat exchange channels 140 would remain similar for both cold hydrogen and cold nitrogen cases, thereby causing little variation in the hydrogen conversion. It can also be seen that the unit cell efficiency remains unchanged between the two cases which is an indication that the convective resistances on the recuperator and heat exchange channels 140, 120 are smaller than that of the thermal resistance along the heat exchange channel 120 (1/{dot over (m)}Cp). Table 4 shows that, similar to cold hydrogen flow, pressure drop in the combustor and recuperator channels 110, 140 are higher when the working fluid flow rate is higher.

TABLE 4 Simulation results for nitrogen as the working fluid H₂ conversion (in {dot over (m)}_(N) ₂ P_(N) ₂ T_(out,N) ₂ combustor) η ΔP (g/s) (bar) (K) (%) (%) (Pa) 26.1 5 233.6 99.9 93.1 5133.7 6.57 5 233.3 99.8 92.4 711.8

Device Level (Physical Layer) Design

As described above, a total of 168 unit cells 100 are needed to increase the temperature of hydrogen flow of 2 g/s from 200 K to 233K. The same amount of heat (911 W) can be removed using 26.1 g/s of nitrogen flow with the same inlet and outlet temperatures. In the present device level design, cold nitrogen gas flow is used for the working fluid. Because the flow rate of cold nitrogen is 13 times larger than that of cold hydrogen, the former presents a limiting case in the design of the headers for uniform flow distribution.

Only fluid flow is simulated at this level due to computational requirements. Properties of nitrogen were estimated at an average temperature of 216.5 K while the properties of hydrogen-air fuel mixture were kept fixed at 400 K. Another important index for performance is pressure drop. The performance measure at the device level pertains to uniform flow distribution amongst the unit cells and an overall low pressure drop within the device.

The microscale combustor and heat exchanger 600 comprising multiple unit cell stacks 400 was designed so it could be fabricated using chemical etching and diffusion bonding, FIGS. 4A-4C, 6. (The device could also be readily manufactured using additive manufacturing technologies such as 3D printing.) The planned diffusion bonding manufacturing method imposed some important limits on the design of individual layers 410-440 which taken together constitute a unit cell stack 400. That is, per the schematic in FIG. 1, it is clear that a single unit cell 100 would have to span five layers in a physical realization of the unit-cell design, that is: upper and lower heat exchange layers 410, 440, upper and lower recuperator layers 420, 430, and a combustor layer 300, FIGS. 3A-4C. The central layer would comprise the combustor microchannel layer 300, which would be surrounded on either side by two recuperator layers 420, 430, each of which in turn would be surrounded by respective heat exchange layers 410, 440.

Since the width of the microchannels 415, 425, 315, 435, 445 (hereafter “415-445”) within each unit cell 100 of the design of FIG. 1 is only 2 mm, several unit cell microchannels 415-445 may be positioned alongside each other in each of the respective individual heat exchanger, combustor, and recuperator layers 410-440, and the layers 410-440 may then be stacked together to form a stack 400 of eight unit cells. That is, each layer 410-440 may include eight constituent channels such that the stack 400 comprises eight unit cells. In this regard, the upper and lower heat exchange layers 410, 440 may each include eight heat exchange channels 415, 445, FIG. 3D; the upper and lower recuperator layers 420, 430 may each include eight recuperator channels 425, 435, FIGS. 3B, 3C; and, combustor layer 300 may include eight combustion channels 315, FIG. 3A. Thus, in combination, the layers 410-440 may provide eight unit cells of overall dimensions of 24 mm×73 mm upon assembly with the remaining recuperator layers 420, 430 and heat exchange layers 410, 440, FIGS. 4A-4C. The thickness of the combustor layer 300 may be 600 μm, with 300 μm deep combustion channels 315 formed via chemical etching. (In fact, all dimensions presented in connection with FIG. 1 may be incorporated in the design of FIGS. 3A-4C.) Two holes, H, may be provided on diagonally opposite corners of the layer 410-440 to permit alignment of the layers 410-440 during diffusion bonding. Following the split catalyst arrangement of FIG. 1, catalyst beds 330, 421, 431 may be provided on the upper and/or lower recuperator layers 420, 430 and combustor layer 300, FIGS. 3A-3C.

In the combustor layer 300, four combustion channels 315 may be located on each side of the combustor layer 300. Locations of different inlets 312 for the combustion gas mixture are also shown in FIG. 3A. The flow direction of each of the streams is shown for one half of the unit cells, FIGS. 4A-4C. It can be seen that the H₂/air mixture and cold nitrogen streams flow in the same direction while the recuperator gases flow counter to the others, FIGS. 4A-4C. The cold nitrogen inlet ports 418, 428, 318, 438, 448 (hereafter “418-448”) may be located on the sides of the layers 410-440 to permit heat gain from the surrounding ambient air, FIG. 4C. The combustion gas inlet ports 412, 422, 312, 432, 442 (hereafter “412-442”) may be located well within the layers 410-440 in order to increase the preheating area. The layers 410-440 may also include respective exhaust outlet ports 416, 426, 316, 436, 446 (hereafter “416-446”) and nitrogen outlet ports 413, 423, 313, 433, 443 (hereafter “413-443”).

In particular, combustion gas inlets 312 may be provided in the combustor layer 300 to introduce a combustion gas into the combustion channels 315, FIGS. 3A, 4A. The combustion gases may pass through the other layers 410, 420, 430, 440 of the cell stack 400 via respective combustion gas ports 412, 422, 432, 442 which do not communicate with respective channels 415, 425, 435, 445 of their respective layers 410, 420, 430, 440, FIG. 4A. Upon entry into the combustion channels 315, the combustion gases may flow across the catalyst beds 330 towards the center of the combustor layer 300. At the innermost end of the combustor channels 315 recuperator passageways 314 may be provided in gaseous communication with the recuperation channels 425, 435 of the adjoining upper and lower recuperator layers 420, 430. Combustion gases may then flow from the recuperator layer 300 through the recuperator passageways 314 to the recuperation channels 425, 435 of the upper and lower recuperation layers 420, 430. In order to reach the recuperation channel 425 of the the upper recuperation layer 420, recuperation passageways 414 may be provided at the innermost ends of the recuperation passageways 425 of the upper recuperator layer 420, FIGS. 3B, 4A. Gases entering the upper and lower recuperation channels 425, 435 from the combustor layer 300 may then travel longitudinally along the recuperation channels 425, 435 from the center to the ends of the layers 420, 430 where the gases may exit through exhaust ports 426, 436, FIG. 4B. The exhaust gases may then exit the cell stack 400 via respective exhaust gas ports 416, 316, 446 of the additional three layers 410, 300, 440. It can be seen that the fuel/air mixture flows in the opposite direction to the recuperator gas flow.

As to the working fluid, the upper and lower heat exchange layers 410, 440 may each include respective working fluid inlets 418, 448 through which a working fluid may be introduced into the respective heat exchange channels 415, 445, FIG. 4C. The upper and lower heat exchange layers 410, 440 may be identical. The working fluid may travel longitudinally along the length of the heat exchange channels 415, 445 toward the center of the heat exchange layers 410, 440 to exit the layers 410, 440 through working fluid outlets 413, 443. The working fluid may then exit the cell stack 400 via respective working fluid ports 423, 313, 433 disposed in the recuperator and combustor layers 420, 300, 430. It should be noted that nitrogen has a much larger flow rate than combustion mixture flow, thereby making flow distribution uniformity a much bigger challenge. Nitrogen may first enter a rectangular outer plenum 418A, 428A, 318A, 438A, 448A, wherein, because of the relatively high velocity of the flow, recirculation occurs. Several small passages 429, 319, 439 may be disposed in a separating wall 427, 317, 437 to let nitrogen enter into a second (inner) resting plenum 428B, 318B, 438B to which the nitrogen gas layers are connected, FIGS. 3A-4C.

The thickness of the five layer stack 400 of eight unit cells may be about 3 mm. Twenty-one such stacks 400 of layers 410-440 may be stacked up with a top and a bottom header caps 602, 604, FIG. 6, to form a 168 unit cell microscale combustor heat exchanger 600 that could provide the desired heat (911 W) to the working fluid. The overall dimension of the layers 410-440, excluding the top and bottom header caps 602, 604 may be 2.4 cm×7.3 cm×6.3 cm. Alternatively, a greater number of unit cells could be located within each layer 410-440 to reduce the height and increase the lateral dimensions of each layer 410-440.

At the layer level, the design ensured that two criteria were satisfied: (1) uniform flow distribution between the microchannels 415-445 within each layer 410-440 and (2) manufacturability. Three-dimensional simulations of fluid flow were used to verify the uniform flow among the recuperator, combustion, and heat exchange channels 415-445. FIG. 5 presents the velocity contours for one half of the combustor layer 300 (i.e., four of the combustion channels 315). Table 5 provides the cross-sectional averaged velocities at each of the four microchannels 315 shown in FIG. 5 of the combustor layer 300, along with the four corresponding heat exchange channels 415 of the upper heat exchange layer 410. Also provided are the cross-sectional averaged velocities for four heat exchange channels 415 of the upper heat exchange layer 410 with nitrogen flowing through them. It is seen that the flow is uniform to within 1 percent in both combustor and upper heat exchange layers 300, 410 indicating uniform flow distribution within each of the layers 300, 410. Note that the flow distribution within the recuperator channels 425, 435 will follow that of the combustor layer 300, since the flow from each combustion channel 315 goes into a corresponding recuperator channel 425, 435.

TABLE 5 Average velocity within each of the channels within a single layer Average Velocities (m/s) Layers Channel 1 Channel 2 Channel 3 Channel 4 Combustor 2.55 2.56 2.57 2.56 layer Heat 37.3 37.4 37.3 37.4 exchange layer

In addition to uniform flow distribution, the design needed to accommodate the diffusion bonding manufacturing requirements. One of these requirements was that the wall thicknesses had to be sufficiently thick to provide a leak free seal between fluids. To meet this requirement, walls that separated different fluids (combustion mixture, exhaust, and cold nitrogen) were thickened to 1 mm. In addition, a conservative 5 mm of solid material was added around the periphery of each layer 410-440 to ensure that fluids did not leak out of the stack 400.

Header Design

Turning to the header, the location of the inlet and exit manifolds 603, 605, 608, 616 of the microscale combustor and heat exchanger 600 were designed to promote uniform flow distribution amongst the different layers 410, 420, 300 430, 440, FIG. 6, where a device comprising several stacks 400 of unit cells is shown. The headering system should be designed in such a way that uniform flow is permitted into each of the fluidic layers 410-440. The headering system may be viewed as including top and bottom header caps 602, 604 along with the respective inlet ports 412-442, 418-448 and outlet ports 416-446, 413-443 of the fluidic layers 410-440.

There are two different fluids flowing in the microscale combustor and heat exchanger 600, therefore there needs to be at least two inlets 605, 608 and two outlets 603, 616. In the current design, for better flow distribution and lower pressure drops in both streams, two inlets 605, 608 are included for each stream. In the present design, hexagonal inlets 416-446 are used to provide better flow distribution and to reduce the size of the headering section, FIGS. 4A-5. The positioning of the exhaust and hydrogen-air mixture manifolds 616, 605 provided for preheating of the hydrogen-air mixture prior to the catalyst location 330 in the combustion channels 315. Moreover, as mentioned before, nitrogen inlet ports 418-448 are located at the outer edges of the microscale combustor and heat exchanger 600 to permit heat gain from the surroundings, and to mitigate the chances of thermal quenching in the combustion channels 315.

Several three-dimensional simulations of fluid flow were performed in FLUENT® V14 (ANSYS, Inc., Cannonsburg, Pa., USA) in order to determine the proper headering design. While not optimized, the designs presented here represent iterative efforts at obtaining uniform flow distribution amongst layers 410-440. In these simulations, 3 sets of 8-unit cell stacks 400 were placed on top of each other to make a 24 unit cell μCHX. The flow rate through each unit cell was kept identical to the largest flow rate 5.75×10⁻⁵ for heat exchange layers 410, 440 and 6.70×10⁻⁶ for the combustor layer 300, and hence the flow distribution for larger stacks should be similar to the one presented herein.

Four additional layers 702, 704, 706, 708 on top and three layers 705, 707, 709 at the bottom of the stacks 400 were necessary to transition from circular tubing of the header caps 602, 604 to the plena that distribute flow amongst different layers 410-440, FIGS. 7A, 7B. The main constraint in designing the headers 602, 604 lay in the transmission of force through the stacks 400 when the pressure is exerted during diffusion bonding. (Micro-texturing was utilized to satisfy the bonding criterion.) Small unsupported gaps between walls of the channels in two adjacent layers were found to be acceptable provided that the ratio on the span to the layer thickness is less than 7. In the current design, this ratio was kept to 5 at the maximum; hence for layer thickness of 600 μm the span was ensured not to exceed 3 mm. The resulting design of the upper four layers 702, 704, 706, 708 of inlet headering and lower three layers 705, 707, 709 is shown in FIG. 7A, 7B.

Pressure contours for flow within the headers 602, 700, combustion gas inlet ports 412-442, and combustion channels 315 is shown in FIG. 8. This figure shows the left half of the combustor layers 300 with the inlet, the manifold and combustion channels (the recuperator and heat exchange channels 415, 445, 425, 435 are not shown. Table 6 shows the average velocity in the 12 channels. It can be seen than the flow is distributed uniformly between layers and maximum difference is less than 4 percent.

TABLE 6 Average velocity magnitude in combustor layers Channel velocity (m/s) Layer 1 2.61 2.59 2.57 2.6 Layer 2 2.57 2.51 2.51 2.57 Layer 3 2.56 2.53 2.53 2.55

Pressure contours of the nitrogen gas flow within the headers 602, 701, nitrogen inlet ports 418-448, and the heat exchange channels 415, 445 are shown in FIG. 9. As with the combustion gas distribution structure 703 and inlet ports 412-442, most of the nitrogen pressure drop occurs in the distribution structure for nitrogen 701 and associated nitrogen inlet ports 418-448. Average velocities in all heat exchange channels 415, 445 in each layer are shown in Table 7. It can be seen than flow is distributed uniformly and maximum difference is less than 6%.

TABLE 7 Average velocity magnitude in nitrogen layers Layer Layer Layer Layer Layer Layer 1 2 3 4 5 6 Velocity 37.2 37.4 37.6 37.4 37.2 37.1 (m/s) 37.1 37.4 37.8 39.4 37.4 37.2 37.1 37.5 37.3 37.2 39.2 37.1 37.2 37.5 37.6 37.1 37.3 37.3

The design presented for the nitrogen headering is an extreme case since the flow rates for nitrogen are 13 times that of hydrogen, which will be the actual working fluid for the device. The thermo-fluidic design presented herein does not consider conjugate heat transfer effects, which could be significant especially when the thermal conductivity of the material from which the layers 410-440 is fabricated is large.

Experimental Results

A prototype of the μCHX 600 of FIGS. 3A-9 was fabricated and the performance characterized for comparison to the results of the design modeling. The prototype comprised 16 unit cells with 8 unit cells in a layer and was capable of producing and transferring about 100 W of thermal energy rate. The device included several stainless steel shims corresponding to layers 410-440 that were chemically etched to form the channels 415-445 of the design, though other materials such as aluminum, or alloys including but not limited to Inconel, Haynes® alloy, or other suitable materials may be used. For example, a combustor shim 300A was fabricated according to the designed combustor layer 300, FIG. 22. Platinum catalyst was deposited selectively within regions of the combustor shim 300A and recuperator shims according to the design of FIGS. 1, 3A-3C to provide high conversion while minimizing the use of the catalyst. The dimensions of the combustor shim 300A were 73 mm in length, 24 mm in width, and 0.75 mm in thickness. In a similar manner, shims were fabricated corresponding to each of the designed recuperator and heat exchange layers 410, 420, 430, 440. The shims were bolted together along with the insulating fluidic headers of the type shown in the designs of FIGS. 6, 7A, 7B.

In the experiments, due to safety considerations, cold nitrogen gas was used as the heat transfer fluid instead of cold hydrogen gas to be used in the actual application. FIGS. 4A-4C show the flow path of H₂/air mixture, exhaust and cold nitrogen flows used in the experiment. The overall size of the 16 unit-cell device was 24 mm×73 mm×11 mm with the weight of 116 g.

An experimental facility 230 was built to characterize the performance of the device, labeled “μCHX” in FIG. 23. Hydrogen and air were premixed before entering the device at point 23A at room temperature while nitrogen stream entered the device at point 23B at a low temperature after exchanging heat to liquid nitrogen within a Dewar. Water vapor was removed from the exhaust gases using a condenser 23C and desiccant filters 23D. Samples were taken from the dry exhaust gases to quantify the amount of uncombusted hydrogen in the exhaust. Hydrogen conversion was defined as the ratio of the amount of hydrogen combusted to that of input hydrogen as given by Eqn. (2) above.

Inlet and outlet temperatures, pressures and flow rates of the streams were measured using digital data acquisition. The gathered data was used to calculate the overall performance of the μCHX and was defined as the ratio of the amount of heat transferred to nitrogen to the chemical energy of input hydrogen as given by Eqn. (1) above, with the work fluid, wf, being nitrogen in the present experiment.

Heat losses were calculated by subtracting the heat transferred to nitrogen from the heat produced in the combustor by the combusted hydrogen.

$\begin{matrix} {Q_{L} = {\frac{{\overset{.}{m}}_{H_{2}}\Delta \; {h_{reaction}\left( {H_{2}{Conversion}} \right)}}{M_{H_{2}}} - {{\overset{.}{m}}_{N_{2}}\left( {h_{out} - h_{in}} \right)}_{N_{2}}}} & (3) \end{matrix}$

The temperature of the body of the device was measured at four different locations. To determine the body temperature, readings from two of the thermocouples that were located closest to the catalyst section of the μCHX were averaged.

Varied parameters include the inlet temperature of cold nitrogen (150-273 K), mass flow rate of cold nitrogen, and mass flow rate of the combustible gas mixture. The performance of the device was characterized using three parameters: pressure drop across the combustor and recuperator channels, hydrogen conversion, and efficiency index. The equivalence ratio, which is defined as the ratio of the molar fuel-to-air ratio at the test conditions to that at stoichiometric conditions, was kept constant at unity.

One of the important factors in hydrogen conversion was the residence time over the catalyst for reactions. Residence time is defined as the time a hydrogen molecule has to react before leaving the catalyst region.

t _(r) =L _(Cat) / V   (4)

where L_(Cat) is the catalyst bed length and V is the averaged velocity of hydrogen/air mixture in a channel. V is calculated by dividing the total flow rate by the number of channels and channel cross section area. FIG. 24 shows the effect of residence time on H₂ conversion and device efficiency. In these experiments, the inlet nitrogen stream temperature was kept constant at −19° C. Table 8 summarizes the parameters of this plot.

TABLE 8 Test conditions related to FIG. 24. Input t_(r) Power H₂ Q_(L) (ms) (W) Conversion η (W) r_(Q) _(L) 55.8 19.8 0.941 0.663 5.49 0.277 49.9 22.1 0.938 0.696 5.33 0.241 27.2 40.5 0.912 0.786 5.11 0.126 11.7 94.3 0.868 0.820 4.51 0.048 8.64 127.2 0.858 0.822 4.57 0.036

It can be seen that H₂ conversion increases by about 10% with increasing residence time from 9-56 ms. However, the efficiency of the device drops from 82% to 66% due to heat losses. When the power generated in a device is small, the effect of heat losses on the overall efficiency of the system can be significant. The device was covered with Pyrogel® and Cryogel® (Aspen Aerogels, Inc., Northborough, Mass., USA) insulations, and the body temperature was kept approximately the same between all experiments at about 186° C. and therefore there was not much difference between heat losses (in watts, see Table 8). However, the ratio of the heat loss to total power ratio which is defined as,

$\begin{matrix} {r_{QL} = \frac{Q_{L}}{{\overset{.}{m}}_{H_{2}}\Delta \; {h_{reaction}/M_{H_{2}}}}} & (5) \end{matrix}$

increased with an increase in residence time. Since the total length of the catalyst and the equivalence ratio were fixed, in order to achieve different residence times the flow rate of the hydrogen/air mixture had to be varied (see Eq. 4). Different hydrogen/air flow rates resulted in different input power to the system (Table 8). Therefore, that heat loss ratio was different for different cases and is shown in FIG. 24. The higher the heat loss ratio, the lower the efficiency of the μCHX.

The body temperature of the combustor was a second important factor in conversion and device efficiency since higher reaction temperatures result in higher reaction rate and hence conversion. FIG. 25 shows an increase in H₂ conversion with an increase in body temperature where body temperature was varied from 124° C. to 196° C. while keeping the residence time the same (9 ms). In these experiments, the inlet nitrogen stream temperature was kept fixed at −70° C.

The heat loss values varied from 1.1 W for the body temperature of 124° C. to 4.6 W for the body temperature of 196° C. Since the power input was high in these cases (120 W), although heat loss increased, the heat loss ratios were small numbers and had insignificant effect on the overall efficiency of the system. Therefore the efficiency also increased with increasing the body temperature. The experimental results showed that hydrogen residence time and body temperature had significant effects on the overall efficiency of the device. Conversions as high as 94.1% and efficiencies as high as 88.3% were achieved, FIGS. 24, 25.

Alternative Design

In another of its aspects, the present invention provides an alternative exemplary configuration of a microscale combustor and heat exchanger (μCHX) 900, which retains the basic features of the unit cell design 100 of FIG. 1, such as the distributed catalyst beds 130, 132, as well as the features of a five-layer structure of FIGS. 4A-4C with a central layer 850 providing the combustion function, two surrounding recuperator layers 830, 870 performing the recuperation function, and two surrounding heat exchange layers 810, 890 providing a heat exchange function, FIGS. 10, 16. Among the chief differences between the two μCHX's 600, 900 are changes to the headering system (FIG. 10), the inclusion of a shroud 500 of inlet gas/fluid used to absorb the heat coming out of the layer stacks 800 (FIG. 17), and the inclusion of pin fins 811, 831, 851, 871, 891 (hereafter “811-891”) in the heat exchanger, recuperator, and combustor layers 810, 830, 850, 870, 890 (hereafter “810-890”), FIGS. 11A-11D. Heat loss from the μCHX 800 by convection is directly proportional to the body temperature, and by radiation is proportional to the fourth power of temperature. By designing a shroud 500 around the layer stacks 800, heat loss from the μCHX 800 is minimized, FIG. 17. In this design, the gas is present in the shroud 500 between a casing 902 and chemically etched/micromachined layer stacks 800 disposed within the casing 902.

Alternative Layer Design

Turning to the layer structure in more detail, consistent with the schematic in FIG. 1, a unit cell stack 800 spans five layers 810-890: the central combustor layer 850 which is surrounded on either side by two recuperator layers 830, 870, which in turn are surrounded on either side by two heat exchange layers 810, 890, FIG. 10. Each layer 810-890 may include two constituent channels such that the stack 800 comprises two unit cells. Specifically, the upper and lower heat exchange layers 810, 890 may each include two heat exchange channels 815, FIG. 11D; the upper and lower recuperator layers 830, 870 may each include two recuperator channels 835, 875, FIGS. 11B, 11C; and, combustor layer 850 may include two combustion channels 855. The combustor, recuperator and heat exchange channels 815, 835, 855, 875, 895 (hereafter “815-895”) include pin fins 811-891, FIGS. 10, 11A-11D. It should be noted that the two unit cells in the cell stack 800 take the place of the eight unit cells of the cell stack 400. Following the split catalyst arrangement of FIG. 1, catalyst beds 840, 860 may be provided on the upper and/or lower recuperator layer 830, 870 and the combustor layer 850 in the base of the channels 835, 855, 875 between and/or on the pin fins 831, 851, 871, FIGS. 10, 11A, 11B, 13. Both upper and lower sides of the combustor layer 850 and upper and lower sides of the upper recuperator layers 830, 870 may be partially covered with the catalyst beds 840, 860. FIG. 13 shows a cross section of the combustor layer 850 with the locations of the catalyst beds 860 between the pin fins 851. Circular pin fins 851 are shown in this design to distribute the flow uniformly and provide support for bonding. However, other pin fin shapes and other methods (for example parallel channels) may also be used for this goal.

Combustion gas inlets 852 may be provided in the combustor layer 850 to introduce a combustion gas into the combustion channels 855, FIG. 11A. The combustion gases may pass through the other layers 810, 830, 870, 890 of the cell stack 800 via respective combustion gas ports 812, 832, 872, 892 which do not communicate with the respective channels 815, 835, 875, 895 of their respective layers 810, 830, 870, 890, FIG. 10. Upon entry into the combustion channels 855, the combustion gases may flow across the catalyst bed 860 around the pin fins 851 towards the center of the combustor layer 850. At the innermost end of the combustor channels 855 recuperator passageways 857 may be provided in gaseous communication with recuperation channels 835, 875 of the adjoining upper and lower recuperator layers 830, 870. Combustion gases may then flow from the recuperator layer 850 through the recuperator passageways 857 to the recuperation channels 875 of the upper surface of the lower recuperation layer 870. In order to reach the recuperation channel 835 of the upper surface of the upper recuperation layer 830, recuperation passageways 837 may be provided at the innermost ends of the recuperation passageways 835 of the upper recuperator layer 830, FIGS. 10, 11B. Gases entering the upper and lower recuperation channels 835, 875 from the combustor layer 850 may then travel longitudinally along the recuperation channels 835, 875 from the center to the ends of the layers 830, 870 where the gases may exit through exhaust ports 834, 874, FIGS. 10, 11A-11C. The exhaust gases may then exit the cell stack 800 via respective exhaust gas ports 814, 854, 894 of the additional three layers 810, 850, 890, FIG. 10. It can be seen that the fuel/air mixture flows in the opposite direction to the recuperator gas flow.

As to the working fluid, the upper and lower heat exchange layers 810, 890 may each include respective working fluid inlets 816, 896 through which a working fluid may be introduced into the respective heat exchange channels 815, 895, FIG. 10, 11D. The upper and lower heat exchange layers 810, 890 may be identical. The working fluid inlets 816, 896 may be located on the sides of the layers 810, 890 to permit heat gain from the surrounding ambient air. (In the case of building heating, the working fluid is the surrounding air; hence there will be no heat gain.) The working fluid travels longitudinally along the length of the heat exchange channels 815, 895 toward the center of the heat exchange layers 810, 890 to exit the layers 810, 890 through working fluid outlets 813, 893. The working fluid may then exit the cell stack 800 via respective working fluid ports 833, 853, 873 disposed in the recuperator and combustor layers 830, 850, 870.

The multi-unit cell stack 800 may be designed such that it may be fabricated using chemical etching and diffusion bonding/laser welding. (The device could readily also be manufactured using additive manufacturing technologies such as 3D printing.) The manufacturing methods impose some important constraints on the design of the layers 810-890 which are addressed in this design. Etching does not provide sharp corners at the bottom of the walls and the cross section of the channels will be U-shaped. That reduces the cross section area and can increase pressure drop and cause maldistribution. In order to reduce the effect of curved corners, the width of the walls should be at least twice the height of the channels 815-895. On the other hand, bonding methods have other limitations. For example for diffusion bonding there should not be an unsupported span, and force should be transmitted from top to bottom in the regions that needs to be diffusion bonded. In addition, diffusion bonding as well as laser welding needs a minimum width for the bonding surface.

Each layer 810-890 may have a thickness of 600 μm, and thus the thickness of the five layer stack 800 may be about 3 mm. Unlike the design of FIGS. 3A-4C, both sides of each layer 810-890 may be etched 150 μm deep. For example, the top surface of the combustor layer 850 and bottom surface of the upper recuperator layer 830 may both be etched to have complementary combustion channels 855 that are 150 μm deep, such that when the combustor layer 850 is sealed to the upper recuperator layer 830 the channels on the lower surface of the upper recuperator layer 830 and on the upper surface of the combustor layer 850 may be joined to form a conjoined combustion channel 855 that is 300 μm tall. Likewise, the lower surface of the combustor layer 850 and upper surface of the lower recuperator layer 870 may both be etched to have complementary recuperation channels 875 that are 150 μm deep, such that when the combustor layer 850 is sealed to the lower recuperator layer 870 the channels on the upper surface of the lower recuperator layer 870 and on the lower surface of the combustor layer 850 may be joined to form a conjoined recuperation channel 875 that is 300 μm tall. Therefore, the bottom side of the combustor layer 850 will have features similar to the upper surface of the bottom recuperator channel which can be seen in FIG. 11C. In a similar manner, the lower surface of upper heat exchange layer 810 may be etched to include features similar to the upper surface of the upper recuperator layer 830, and the lower surface of lower recuperator layer 870 may be etched to include features similar to the upper surface of the lower heat exchange layer 890.

There are two reasons for having 150 μm deep features on both sides instead of 300 μm deep features on one side. The first reason pertains to the constraints of the fabrication process. Isotropic chemical etching limits the distance between features to be at least twice the etching depth. For example, if the etch depth is 300 μm, the distance between features has to be at least 600 μm. However, in the designs in FIGS. 11A-11D, the distance between the pins 811-891 may be as small as 400 μm in order to permit uniform flow distribution. Therefore an etching depth of 150 μm is chosen. The other reason is that, as shown in FIG. 1, both sides of the combustor and upper recuperator layers 850, 830 are to include catalyst beds 840, 860. Without an etched surface on the back side of the layers 850, 830, the catalyst deposition would be done on a smooth surface, which does not permit good adherence of the catalyst to the layers 850, 830. Even if the flat surface were etched, catalyst would cover the entire flat surface making bonding of the layers 850, 830 impossible due to the catalyst layer on one layer 850, 830 being in contact with the pin 831, 851 on the adjoining layer. Existance of features such as pins 831, 851, on the lower sides of the layers 830, 850 lets the catalyst solution flow around the features keeping the bonding contact surface clean, FIG. 13. Other fabrication techniques may require different design considerations, or the fabrication technique may not put any constrain on the features or the channel height, in this case the two sides of the layers 810-890 may have features with different heights.

In addition to uniform flow distribution, the design needed to accommodate the bonding manufacturing requirements. One of these requirements was that the layer wall thicknesses had to be sufficient to provide a leak free seal between fluids. To meet this requirement, walls that separated different fluids (combustion mixture, recuperator, and cold nitrogen) were thickened to at least 2 mm. In addition, a conservative 3 mm of solid material was added around the periphery of each layer 810-890 to ensure that fluids did not leak out of the stack 800.

At the layer level, the design again ensured that two criteria were satisfied: (1) uniform flow distribution between the microchannels 815-895 within each layer 810-890, and (2) manufacturability. Three-dimensional simulations of fluid flow were used to verify the uniform flow among the channels 815-895. FIGS. 14A, 14B present the velocity contours for one half of the combustor and heat exchanger layers 850, 810, 890, respectively. FIGS. 15A, 15B show the velocity magnitude plots for combustor and heat exchange layers 850, 810, 890, respectively, taken along the dashed lines in FIGS. 41A, 41B. It can be seen that uniform flow distribution is achieved.

Shrouding and Headering

Heat loss from a combustor by convection is directly proportional to the body temperature. By designing a shroud 500 around the cell stack 800, heat loss is minimized. Specifically, the cell stacks 800 may be stacked together to provide four unit cells which may then be placed in an enclosing casing 901 comprising lower and upper casing portions 902, 904 which are larger than the stacks 800, FIGS. 16, 17. Thus, the space between the relatively smaller stacks 800 and the larger lower casing portion 902 may provide a gap between the stacks 800 and lower casing portion 902 to provide the shroud 500, FIG. 17. The shroud 500 represents one option for distribution of the working fluid around the cell stacks 800 and inside the heat exchange layers 810, 890. The distance between the sides of the cell stacks 800 and the casing 901 internal sides may be found by numerical simulations. The distance on the long side (along the length) may be 1 mm and on the short side may be 3 mm, for example. (Since the casing 901 is separated from the hot portions of the cell stack 800, the casing 901 can be made of lower temperature materials including, but not limited to aluminum, wood, plastic, and so forth.)

The heat exchange fluid may flow in the shroud 500 around all the layers 810, 890 that have openings 816, 819 for the intake of the working fluid. In this regard there may also be a space between a top distribution layer 920 and the layer stacks 800 and also between the lower casing portion 902 and the layer stacks 800 which lets the working fluid flow around the layer stacks 800. Therefore in addition to the sides, there is working fluid on top and bottom of the layer stacks 800. It can be seen from FIGS. 10, 17 that the inlets 816, 896 of the heat exchange layers 810, 890 are extended to the edge so that the working fluid can enter the layers 810-890 from the casing 901. The rest of the working fluid stream enters the stacks 800 via the working fluid outlets 813, 833, 853, 873, 893 (hereafter “813-893”) at the center of the layers 810-890 and mixes with the hot working fluid. Flow path for different streams are shown in FIGS. 10, 17. The casing 901 may include working fluid inlets 908, combustion mixture inlets 905, recuperator gas outlets 916, and working fluid outlet 903 to assist in creating the different flow streams, FIG. 16. In addition, upper and lower distribution layers 920, 920A, 922 disposed between the casing 901 and stacks 800 may include working fluid inlets 928, combustion mixture inlets 925, 925A working fluid outlet 923, 923A, and recuperator gas outlets 926, as well as alignment pin holes H, to further assist in directing the flow streams, FIG. 17. For example, the gas enters from the bottom distribution layer 922 to the center plenum (i.e., working fluid outlets 813-893) and mixes with the hot streams coming out of the heat exchange layers 810, 890.

The flow distribution arrangement between the shroud 500 and the heat exchange channels 815, 895 causes only a fraction (typically ⅕th) of the fluid, to enter the heat exchange channels 815, 895. Fluid going through these channels 815, 895 gets heated by heat transfer from the recuperator channels 835, 875 and exits at temperatures typically in excess of 100° C. The hot fluid then mixes with the bypassed fluid in the casing, and the desired air temperature is attained at the exit 903 of the μCHX 900. The distance between the lower casing portion 902 and the bottom of the cell stacks 800 may be adjusted, such as by raised spacers which may be part of the recuperator gas outlets 926, such that the desired mixture ratio is achieved (e.g., 600 μm in this case). This flow distribution arrangement is important for two reasons: (i) by allowing only a fraction of the inlet working fluid to enter the heat exchange channels 815, 895, the overall body temperature within the combustion, recuperation and heat exchange channels 815-895 is maintained relatively high, thereby resulting in high reaction rates and high fuel conversion; and, (ii) the pressure drop through the μCHX 900 is reduced when compared with the entire fluid flowing through the heat exchange channels 815, 895.

To further supplement the insulative effect of the shroud 500, one or two sets of concentric grooves 818, 838, 858, 878, 898 (hereafter “818-898”) may be positioned around the periphery of each of the layers 810-890 exterior to the respective channels 815-895 to further reduce the heat transfer out of the cell stack 800, FIGS. 10-11D. The grooves 818-898 located around the combustor, recuperator and heat exchange channels 815-819 to separate these channels 815-819 from the shroud 500. The grooves 818-898 may help reduce the axial conduction of heat from the combustor region to the shroud 500. In the absence of these grooves 818-898, the working fluid in the shroud 500 may cool down the combustor region thereby reducing the reaction rate and hence fuel conversion. Since the layers 810-890 may comprise a thermally conductive material that permits conduction of heat from one or more of the channels 815-895 to the shroud 500, inclusion of the grooves 818-898 containing material, such as a gas or liquid, (or absence of the material, such as a vacuum) that is less thermally conductive than the layers 810-890 may deter heat conduction to the shroud 500. Furthermore, the grooves 818-898 may include through holes 819, 839, 859, 879, 899 to further decrease the heat transfer rate by reducing the amount of material in the layers 810-890 that may add to the heat transfer area (FIGS. 11A-12B). Thus, the combination of the shroud 500 and the grooves 818-898 cooperate to reduce heat loss (high device efficiency) and high conversion, respectively, and is a salient feature of this exemplary configuration.

Several three-dimensional simulations of fluid flow were performed in FLUENT® V14 (ANSYS, Inc., Cannonsburg, Pa., USA) in order to determine the proper headering design. Pressure contours for flow within the heat exchange layers is shown in FIG. 18. This figure shows the flow field for a quarter of the device shown in FIG. 17 with nitrogen as the working fluid. The velocity vector in the mixing region, shown in FIG. 19, indicates that the upward flow through the mixing region does not enter the channels and cause flow reversal.

A comparison between the pressure drop between the designs of FIGS. 4A-4C and FIG. 10 is summarized in Table 9. In this table the pressure drops at layer and stream levels are shown. The stream level pressure drops includes pressure drops in the layers and headers. It can be seen that mass flow rates are kept the same for all three different designs. Nitrogen was used as the working fluid in these simulations. The pressure drop in the fuel/air stream is much lower than that of the working fluid, because the flow rates are much smaller. In the layer level, pressure drop in the combustor layer 850 of FIG. 10 (0.200 kPa) is higher than that of the combustor layer 300 of FIGS. 4A-4C (0.036 kPa), due to the existence of pin fins 811-891. At the header level, on the other hand, the pressure drop of the fuel/air stream is much smaller for the FIG. 10 design (0.201 kPa) compared to the FIGS. 4A-4C design (1.4 kPa). Only 1 Pa pressure drop exists in the headering section of the FIG. 10 design.

An important achievement here is the reduction of pressure drop in the heat exchange stream. The pressure drop in the FIG. 10 design is 7 times less in the layer level and more than 10 times less in the stream level. Therefore a significant improvement is achieved with regard to pressure drops.

TABLE 9 Comparison of pressure drops between different designs Pressure drop (kPa) mass flow Section Gen II Gen III rates (kg/s) Heat exchanger layer 37 5 5.75E−05 Heat exchange 171 13 2.53E−03 stream Combustor layer 0.036 0.200 6.70E−06 Fuel/air stream 1.4 0.201 1.33E−05

Other Possible Design

In yet a further possible design 200 in accordance with the present invention is incorporation of the casing 901 and shroud 500 of FIGS. 10, 17 within the layers 250, FIGS. 20, 21. With this design 200, the size may be smaller; however, pressure drop may be higher. FIG. 20 shows the location of a shroud 210 disposed directly within the layers 235, 250 which replaces the role of the casing. The shroud 210 may be provided in the form of one or more passageways that extend through each layer 250. The combustor, recuperation, and heat exchange channels 240 are thus encased within a shroud 210 of inlet gas/fluid that is to be heated using the device 200. FIG. 21 shows an exemplary combustor layer 235 for this new design 200. A circumferential grove 220 may exist to reduce heat transfer from the shroud 210 to the channels 240.

Electric Heating Option

In another aspect of it aspects, the present invention may provide a microscale combustor 2600, 2601 which includes an auxiliary heat source to heat a selected portion of the combustion channel 2610, 2620 to assist in initiating a catalytic reaction within the combustion channel 2610, 2620. For example, the combustion channel 2610, 2620 may be included a part of a device having the configuration shown in FIG. 1. The heat source may be provided in the form of a wire 2604 disposed in thermal communication with a wall 2613 of the combustion channel 2610, with the wire electrically connected to a power source 2602 such that the flow of electricity through the wire 2604 heats the wire 2604 and the wall 2613 and any catalyst disposed proximate thereto. Alternatively the heat source may be provided within a portion 2624 of the wall 2623 of the combustion channel 2620, with the wall portion 2624 electrically connected to a power source 2622 such that the flow of electricity through the wall portion 2624 heats the wall portion 2624 and any catalyst proximate thereto. Thus, the present invention may provide a microscale combustor comprising plurality of layers each comprising one or more respective channels extending therethrough. The layers may be joined to one another to permit gaseous communication between selected respective channels of the layers, with the plurality of layers comprising a combustor layer having at least one combustion channel with a catalyst disposed therein. In addition, one or more electric heaters may be provided at a selected location to heat at least a portion of the combustion channel to initiate the catalytic reaction. The electric heater may include a wire or may extend along and/or through an entire surface of the combustion channel, for example. The electric power may be provided from external sources or from a rechargeable battery located close to or attached to the combustor.

These and other advantages of the present invention will be apparent to those skilled in the art from the foregoing specification. For example, while the exemplary configurations illustrate parallel flow among the channels of the layers, cross flow channel configurations may also be utilized in accordance with the present invention. Accordingly, it will be recognized by those skilled in the art that changes or modifications may be made to the above-described embodiments without departing from the broad inventive concepts of the invention. It should therefore be understood that this invention is not limited to the particular embodiments described herein, but is intended to include all changes and modifications that are within the scope and spirit of the invention as set forth in the claims.

The contents of all publications cited throughout the text of this disclosure are hereby incorporated herein by reference. In addition, the following symbols used throughout this disclosure have the following meanings:

A Pre-exponential factor (Arrhenius equation) (mol-cm-K-s)

Cp Specific heat at constant pressure (J/kg·K)

E_(a) Activation energy (Arrhenius equation) (kJ/mol)

h Enthalpy (J/kg)

H_(c) Combustion channel height (m)

H_(o) Working fluid channel height (m)

H_(R) Recuperator channel height (m)

M Molar mass (kg/kmol)

{dot over (m)} Mass flow rate (kg/s)

P Pressure (Pa)

T Temperature (K)

Y_(g) Mass fraction of g^(th) gaseous species

Greek Symbols

β Temperature exponent (Arrhenius equation)

φ Species equivalence ratio in a reaction

Γ Surface site density (mol/cm²)

η Efficiency of the unit cell

Subscripts

0 Value at the inlet

in Inlet

g Index for species

out Outlet 

What is claimed is:
 1. A microscale combustor and heat exchanger, comprising: a plurality of layers each comprising one or more respective channels extending therethrough, the layers joined to one another to permit gaseous communication between selected respective channels of the layers, the plurality of layers comprising: a combustor layer comprising at least one combustion channel having a catalyst disposed therein; and a recuperator layer comprising at least one recuperation channel having a catalyst disposed therein, the at least one recuperation channel disposed in gaseous communication with a respective one of the at least one combustion channel to receive a combustion gas therefrom.
 2. The microscale combustor and heat exchanger according to claim 1, wherein the combustor layer and recuperator layer are disposed in stacked arrangement so that the at least one recuperation channel and the at least one combustion channel are disposed over one another with a common channel wall therebetween.
 3. The microscale combustor and heat exchanger according to claim 2, wherein the at least one recuperation channel and at least one combustion channel are oriented relative to one another such that the catalyst containing portion of the at least one recuperation channel is adjacent the catalyst containing portion of the at least one combustion channel.
 4. The microscale combustor and heat exchanger according to claim 1, comprising an additional recuperator layer comprising at least one recuperation channel disposed therein disposed in gaseous communication with a respective one of the at least one combustion channel to receive a combustion gas therefrom.
 5. The microscale combustor and heat exchanger according to claim 4, wherein the at least one recuperation channel of the additional recuperator layer has a catalyst disposed therein.
 6. The microscale combustor and heat exchanger according to claim 1, wherein one or more of the at least one recuperation channel and at least one combustion channel comprises pin fins disposed therein.
 7. The microscale combustor and heat exchanger according to claim 1, wherein the plurality of layers includes a heat exchange layer comprising at least one heat exchange channel disposed therein and wherein the recuperator layer is disposed between the heat exchange layer and the combustor layer.
 8. The microscale combustor and heat exchanger according to claim 1, wherein one or more of the plurality of layers comprises a shroud disposed between the one or more respective channels and an edge of the layer.
 9. The microscale combustor and heat exchanger according to claim 8, wherein the shroud includes a passageway extending through the layer in which the shroud is disposed.
 10. The microscale combustor and heat exchanger according to claim 8, wherein one or more of the plurality of layers comprises a groove disposed therein between the shroud and an edge of the layer.
 11. The microscale combustor and heat exchanger according to claim 1, wherein one or more of the plurality of layers comprises a groove disposed therein between the one or more respective channels and an edge of the respective layer.
 12. The microscale combustor and heat exchanger according to claim 1, wherein ratio of the area of catalyst in the combustion channel to the area of catalyst in recuperation channel is the range of 1/10 to 9/10.
 13. The microscale combustor and heat exchanger according to claim 1, wherein ratio of the amount of catalyst in the combustion channel to the amount of catalyst in recuperation channel is the range of 1/10 to 9/10.
 14. A microscale combustor and heat exchanger, comprising: a plurality of layers each comprising one or more respective channels extending therethrough, the plurality of layers comprising a combustor layer having at least one combustion channel disposed therein, and a heat exchange layer having at least one heat exchange channel disposed therein; and a casing disposed around and enclosing the plurality of layers and comprising an inner wall defining a cavity disposed therein, the cavity dimensioned to provide a gap between at least a portion of the inner wall and the plurality of layers, wherein the at least one heat exchange channel is disposed in gaseous communication with the gap.
 15. The microscale combustor and heat exchanger according to claim 14, wherein the plurality of layers includes a recuperator layer comprising at least one recuperation channel disposed therein, and wherein the recuperator layer is disposed between the heat exchange layer and the combustor layer.
 16. The microscale combustor and heat exchanger according to claim 15, wherein the combustor layer and the recuperator layer are disposed in stacked arrangement so that the at least one recuperation channel and at least one combustion channel are disposed over one another with a common channel wall therebetween.
 17. The microscale combustor and heat exchanger according to claim 15, wherein one or more of the at least one combustion channel, at least one recuperation channel, and at least one heat exchange channel comprises pin fins disposed therein.
 18. The microscale combustor and heat exchanger according to claim 15, wherein the at least one recuperation channel has a catalyst disposed therein.
 19. The microscale combustor and heat exchanger according to claim 14, wherein the at least one heat exchange channel includes an inlet in gaseous communication with the gap and wherein the recuperator and combustor layers each include respective working fluid outlet ports in gaseous communication with the gap.
 20. The microscale combustor and heat exchanger according to claim 19, wherein the inlet of the heat exchange channel is disposed at an edge of the heat exchange layer.
 21. The microscale combustor and heat exchanger according to claim 14, wherein one or more of the plurality of layers comprises a groove disposed therein between the one or more respective channels and an edge of the respective layer.
 22. The microscale combustor and heat exchanger according to claim 21, wherein the groove comprises a through hole extending through the layer in which the groove is disposed.
 23. The microscale combustor and heat exchanger according to claim 14, wherein one or more of the plurality of layers comprises two concentric grooves disposed therein, the grooves disposed between the one or more respective channels and an edge of the respective layer.
 24. The microscale combustor and heat exchanger according to claim 14, wherein the at least one combustion channel has a catalyst disposed therein.
 25. The microscale combustor and heat exchanger according to claim 14, wherein at least one of the plurality of layers comprises an upper surface and a lower surface each of which surfaces includes a respective channel disposed therein.
 26. A microscale heat exchanger, comprising: a plurality layers each comprising one or more respective channels extending therethrough, the plurality of layers comprising one or more heat exchange layers having at least one heat exchange channel disposed therein, the plurality of layers disposed in stacked arrangement so that at least one channel from each of two or more layers is disposed adjacent one another with a common wall therebetween through which heat may be exchanged, wherein the plurality of layers comprises a shroud disposed between the one or more respective channels and associated respective edges of the layers.
 27. The microscale heat exchanger according to claim 26, wherein the plurality of layers includes a combustor layer comprising at least one combustion channel.
 28. The microscale heat exchanger according to claim 26, wherein one or more of the plurality of layers comprises a groove disposed therein between the shroud and an edge of the layer.
 29. The microscale heat exchanger according to claim 26, wherein the shroud includes a passageway extending through the layer in which the shroud is disposed.
 30. The microscale heat exchanger according to claim 26, wherein one or more of the plurality of layers comprises a groove disposed therein between the shroud and an edge of the layer.
 31. The microscale heat exchanger according to claim 30, wherein the groove comprises a through hole extending through the layer in which the groove is disposed.
 32. A microscale combustor, comprising: a plurality of layers each comprising one or more respective channels extending therethrough, the layers joined to one another to permit gaseous communication between selected respective channels of the layers, the plurality of layers comprising a combustor layer comprising at least one combustion channel having a catalyst disposed therein; and an electric heater disposed in thermal communication with the at least one combustion channel. 